This invention relates generally to minimizing the amount of Electro Magnetic Interference (EMI) radiation produced by electronic devices, and more specifically, to low cost active and passive planar EMI shields for high frequency electronic devices having attached heatsink members.
Generally, electronic devices emit electromagnetic radiation as an unwanted byproduct of their operation. Electronic devices are also sensitive to external electromagnetic radiation which may corrupt internal signals. Thus, electronic device operation may be interfered with by the action of outside electromagnetic radiation. This is known as Electro Magnetic Interference, or EMI. It is necessary to protect electronic devices from external sources of electromagnetic energy/EMI, and also to prevent internal electromagnetic energy from escaping and possibly interfering with other surrounding electronic devices. One technique used to protect electronic devices from EMI is to surround a system containing the electronic device or devices with a grounded Faraday cage known as a shield. While this approach is effective to reduce EMI levels for an electronic system, it is not effective for individual components within a system. The Faraday cage or shield is often made as a part of the physical enclosure or cabinet of the electronic system. Also, such Faraday cage shielding may be expensive.
Electronic devices are also known to produce heat as an unwanted byproduct of their operation. An increase in temperature of an electronic device may result in a decrease in device reliability and functional lifetime. Thus it is often found necessary to cool electronic devices. One method known in the art for cooling electronic devices is to have a device known as a heatsink attached to the electronic device. The heatsink conducts heat from the electronic device to a source of cooler fluid, such as outside air to reduce the temperature of the electronic device. Typically, the surface area of the heatsink is larger than the surface area of the electronic device itself in order to improve the efficiency of the heat transfer to the cooling fluid. A problem exists in the art because the increased area of the heatsink may also act to increase the amount of EMI emitted by, or absorbed by the electronic device because the heatsink may act as an antenna.
Thus while it is desirable to maximize the total surface area of the heatsink, since this improves cooling efficiency and increases device reliability and potential lifetime, such a maximization of heatsink efficiency may also increase the unwanted emission of EMI radiation from the electronic device.
One type of heatsink that is thermally very efficient is known as a unidirectional heatsink. A unidirectional heatsink typically has fins oriented in a vertical direction so that the air flow is confined to one direction. Unidirectional heatsinks are typically more efficient than omnidirectional heatsinks because the path length that the heat must travel from the IC to the cooled surface is shorter. Unfortunately, the addition of any heatsink generally increases the efficiency of the unwanted radiation.
The heatsink does not need to have a direct physical contact to the electronic device for the heatsink to act as an unwanted antenna. Electronic devices such as integrated circuits (ICs) operate at such high frequencies that even an electrically isolated metal heatsink may become `capacitively coupled` to the IC, and thus still act as an antenna. This would occur if the spacing between the IC and the antenna is small enough.
A high performance IC is typically packaged in a hermetic ceramic package with a thin electrically insulating ceramic wall between the backside of the IC and the heatsink. For a state of the art high speed and high powered IC the thin ceramic wall produces too much thermal resistance for the amount of heat the IC generates. Such a high powered IC would typically be packaged in a ceramic package that has the metal heatsink penetrating completely through the ceramic package to reduce the resistance to heat flow, and thus a direct electrical connection from the heatsink to the backside of the IC exists.
It is known in the art that the emission of radiation from heatsinks may be reduced by electrically grounding the heat sink. This approach has problems however, such as the possibility of the ground connection producing what is known as a reverse biased junction on the IC. Reverse biased junctions can cause an IC, in particular what are known as CMOS circuits, to go into a destructive state such as latch-up, which can cause the IC to completely destroy itself.